Explaining the Day of Decision and what occurs during it

Allah says about the Day of Decision -- and it is the Day of Judgement -- that it is at a fixed time, with a set appointment. Its time cannot be added to or decreased. No one knows its exact time except Allah. This is as Allah says,

وَمَا نُؤَخِّرُهُ إِلاَّ لاًّجَلٍ مَّعْدُودٍ

إِنَّ جَهَنَّمَ كَانَتْ مِرْ‌صَادًا (Surely Jahannam [ the Hell ] is set in ambush. 78:21). The word mirsad means 'an ambush' or 'lying in wait to attack unawares' or 'a secret position for surprise attack'. Here Hell refers to the bridge of Hell. The angels of reward and punishment will lie in wait. The angels of punishment will make a surprise attack on the inmates of Hell, and the angels of reward will lie in wait to accompany the inmates of Paradise and take them to their abode. [ Mazhari ]

Sayyidna Hasan Basri (رح) said that there will be an outpost of guardian angels on the bridge of Hell. If anyone has a permit to enter Paradise, he will be permitted to enter; but if anyone does not have the permit to enter Paradise, he will be prevented from entering it. [ Qurtubi ]